滇西哀牢山构造带变质岩系LA-ICP-MS锆石U-Pb年龄及其地质意义

LA-ICP-MS zircon U-Pb dating of the metamorphic rocks in the Ailaoshan tectonic belt, western Yunnan, and its geological implications

为厘定哀牢山岩群的时代和大地构造归属,选取哀牢山岩群中的花岗质片麻岩(SM-15)、花岗闪长质片麻岩(SM-22)、同变形的花岗质岩脉(SM-18)及紧邻哀牢山岩群的变质砂岩(NO-1)为研究对象,运用LA-ICP-MS锆石U-Pb定年法对其进行定年。结果显示,样品SM-15中有岩浆锆石和变质锆石2类,岩浆锆石的年龄集中于722~740Ma之间,变质锆石的^(206)Pb/^(238)U年龄加权平均值为29.9±0.5Ma。样品SM-22中2组^(206)Pb/^(238)U年龄加权平均值,原岩年龄为232.3±4.3Ma;后期变质重结晶年龄为29.2±0.4Ma。样品SM-18中锆石^(206)Pb/^(238)U年龄加权平均值为27.19±0.36Ma,代表变质年龄。样品NO-1中的碎屑锆石年龄分4组:246~276Ma、600~800Ma、800~1000Ma和1000Ma以上。以上年龄数据表明,哀牢山岩群中至少包括元古宙岩浆岩(722~740Ma)、二叠纪末—三叠纪初岩浆岩或地层(235~255Ma),以及新近纪岩浆岩(27~32Ma),是一个复杂的变质岩带,而不是以往认为的全部属于元古宙扬子地台的结晶基底。哀牢山岩群的主变质时代集中在27~32Ma之间,哀牢山岩群现今的面貌主要在这一时期成型。

In order to determine the age of Ailaoshan Group and its tectonic affinity, the authors chose granodioritic gneiss（SM-15 and SM-22）, its synchronic deformational granite vein intrusion（SM-18） in the high-grade metamorphic rock series and its proximate meta-sandstone（NO-1） in the low-grade metamorphic rock series as research objects and present new results of zircon U-Pb dating of four samples so as to do the accurate chronology tests. Two groups of ages were identified in sample SM-15 and SM-22. In sample SM-15, the older magmatic group yielded an age of 722~740Ma and the metamorphic younger group yielded an age of 29.9±0.5Ma. In sample SM-22, the older is 232.3±4.3Ma and the younger is 29.2±0.4Ma. In addition, the zircons from sample SM-18 yielded a concordant U-Pb ages of 27.19±0.36 Ma. A total of 30 spots analyses of sample NO-1 defined four age groups at 246~276Ma, 600~800Ma, 800~1000Ma and 1000Ma respectively. From these data, the authors have drawn some conclusions： Firstly,the Ailaoshan Group consists of at least three stages of tectonic and magmatic activities, i.e., the 722-740 Ma and 235±10Ma igneous rocks and sedimentary rocks, and the 27-32 Ma igneous rocks. The metamorphic rocks in the study area are unlikely to be of the Yangtze Plate; Secondly, the peak metamorphism deformation time of Ailaoshan Group was about 27-32 Ma.